Semi-conductor devices employing cadmium telluride



D. A. JENNY Dec. 24, A1957 SEMI-CONDUCTOR DEVICES EMPLOYING CADMIUM TELLURIDE Fild Nov. 25. 1953 my r IN1/Emma `f BY i WORNEY lidi' United States Patented Dec. 24, 1957 SEMI-CONDUCTOR DEVICES EMPLOYING CADMIUM rIELLURIDE Dietrich A. Jenny, Princeton, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application November 25, 1953, Serial No. 394,391

s cintas. (c1. 317-237) This invention relates toimproved semi-conductor materials, devices and methods of making them. More particularly, the invention relates to devices utilizing semiconductive cadmium telluride, to methods of making ptype semi-conductive cadmium telluride and to methods of forming p-n rectifying barriers or junctions in bodies of' n-type semi-conductive cadmium telluride.

Many different semi-conductor devices utilizing semiconductive germanium and silicon are known. The op` eration of these devices is` usually subject to `relatively severe maximum temperature limitations. The temperature limitation of a typical device is determined` primarily by the energy gap between the valence band and the conduction band of the semi-conductive material of the device. When the temperature of the device is increased to a point where thermal energy becomes `sufcient `to raise substantial numbers of electrons across the energy gap, the semi-conductive characteristics of the material are adversely affected. For example, the energy gap` of Accordingly it is an object of the instant invention to e provide improved semi-conductive materials.` j `Another object is to provide improved semi-conductive materials having relatively high energy gaps between their conduction and valence bands. l Another object is to provide improved semi-conductor devices operable at relatively high temperatures. `1 Another object is to provide improved semi-conductive cadmium telluride having p-type conductivity. j l Another object is to provide within a body of n-type semi-conductive cadmium telluride a region of p-type conductivity and a p-n rectifying junction between said region and the remainder of the body. l Still another object is to provide a method of creating a p-n rectifying junction within a body of n-type semiconductive cadmium telluride.

` A still further object is to provide an improved method of forming an electrode fused to a body of semi-conductive cadmium telluride having a p-n rectifying junction associated therewith. f

These and other objects may be accomplished by the practice of the instant invention according to which it has now been discovered that elements of the Ia and `Va columns of the periodic table yield p-type impurity cen-l ters when dispersed in semi-conductive cadmium telluride. It has also been found that small bodies of these elements, or of alloys including substantial proportions of them, may be fused to n-type semi-conductive cadium telluride to form electrodes having p-n `rectifying junctions. The energy gap of cadmium telluride is about 1.45 electron volts, and devices based on cadmium telluride areoperable at relatively high temperatures. t

The invention will be described in greater detail in connection with the drawing of which: Figure l is a partially schematic, elevational, crosssectional View` of apparatus suitable for use in producing semi-conductive cadmiumtelluride according to theinstantinvention Y e Figures 2 and 3 areschematic, elevational, cross-sctional views of `a`wafer of semi-conductive cadmium telluride illustrating successive steps in the production of a semi-conductor device according tothe invention. e

Similar reference` characters are appliedto similar elements throughout the`dr`awing.

One lfeature of the instant invention is the production of p-type semi-conductive cadmium telluride, which may be conveniently made utilizing the apparatusshown in Figure `l. `A `quantity 2,-for example, aboutlO gms.`,"`of relatively pure cadmiumitelluride isplacedwithinarefractorytubell which maybe of quartz. In place of cadmium tellurideequivalent quantities of cadmium and tellurium yin stoichiometric proportions may be used. About 0.1% `by wgt. of `animpurity-yielding material `is placed in the tube with= the cadmiumttelluride. t This material may be any of the elements ofthe la and Va Groups of the Periodic Table according to Mendeleeil. (By the Ia group is meant the group including lithium, sodium, potassium, rubidium and cesium. ThefVa group includes nitrogen, phosphorus,arsenic, antimony and bismuth.) Conveniently, antimony may be used. i

Generally, any quantity in excess of l0"6 atomic percent of one or` more, of the` elements'specied issufcient to impart p-type conductivity to semi-conductive cadmium telluride. `It. is preferred not to `employ relatively large quantities, thatis, over a few percent, of any ofthe impurity-yielding materials since the semi-conductivecharacter of the cadmium telluride may be adversely affected. If, for example, too much arsenic is addedl a ternary compound of cadmium, arsenic` and tellurium may be formed, or an alloy of cadmiumjztelluride andcadmium arsenideor, `in general, a poly-phase alloy which lacks desirablesemi-conductive characteristics. j t t.

The tube is evacuated `and sealed o-fl `to prevent `contamination of the material andloss of `any of its constituents by vaporization. The tube is placed within` a furnace 6 which may be heated by` any convenientmeans such as the electric resistance elements 8.,.lnitially `the tube is centered longitudinally mwithinthe furnaceland is` heated to about l C. completely to melt the `cadmium telluride. The material is` maintained in a molten state for at least a fewminutes to' insure `complete intermin-` ture of the cadmium, `telluriuin and the impurity-yielding material. i e e i.

The cadmium telluride is then frozen progressively from one end of the tube to theother by a gradient freezing process. This may be accomplished by slowly Withdraw.-` ing` the quartz tubefrom the furnacefat a controlled rate. Preferably, however, to` produce a` more uniform ingot the tube is mbveda short distance within the furnace `to an asymmetrical positionl with respecttofthe heating elements. The effect of this change of position, to approXirnately the location illustrated, is to provide ateniperature gradient along the length of the charge. One

end 12 of the charge is removed from the directfh'eating` effect of the resistance elements and ismaintained at a, slightly lower temperature than the remainder offthe is reduced. `lf a relativelysteep temperature gradientl is provided, the uniformity of thefrozen ingot may be `af,-y fected `by preferential condensation of vapors at the "low temperature end of the tube.

"This process is called gradient freezing and may be accomplished by any knownA apparatus. Gradient freezing assists in producing a `cadmium telluride ingot a major portion of which has a relatively high degree of purity. Many impurities and also a'substantial .proportion of the yimpurity-yielding.material initially added to themelt are'driven by the gradient freezing process to the lastfrozen portion of the ingot. This eifect depends yof course upon the segregation characteristics of the system.

A cadmium telluride ingot `thus produced comprises several relatively large crystallites, some of which may be as large as about 1/3v cubic centimeter. Substantially allot the ingot exhibits p-type `semi-conductivity due 'primarily to the impurity centers introduced into the material by the element of -column Ia or Va. This is unexpected and surprising since in previously known semiconductive materials such as germanium, impurity elements .of the Iaand Va columns are known to yield ntype conductivity. The apparently opposite effect in cadmium telluride is explainable on a basis of atomic substitutions in the crystal structure. It is not intended to limit the scope of the invention by this theory. It is believed, however, that the elements of column Ia substitute themselves in the crystal` lattice of cadmium telluride more readily in the place of cadmium than in the placeof tellurium. Since cadmium has two valence electrons and the elements-ofthev Ial group have only one Valence-electron, one electron for each impurity atom is missing. in the crystal lattice. Such missing electrons are called holes and act as positive electriccharge carriers. vIn the case of the group Va elements `it is believed that these are substituted in `the place of tellurium in the crystal lattice, again with the creation of hole lcharge carriers.

Antimonyis a preferred material for` use in doping cadmium telluride according tothe invention. Junction devices made utilizingI antimony doped cadmium telluride have higher rectification ratios `than similar devices made with other doping material. For use in making relatively low resistivity cadmium telluride, however, lithium is a preferred material according to theA invention.

From a chemical point of vieu/the materials of the invention may be considered to be cadmium telluride containing traces of a group vIa tellurideror ofa cadmium compound of a g1oup Vz.element.. y" lhis analysis is believed proper from va chemical standpoint, but lsince the impurityelement atoms aretprimarilydeterminative of the conductivity type of the material, it is preferred to consider these atoms alone.

'An valternative method of introducing Ygroup la element impurity 4centers Iinto cadmium telluride.,comprises melting eadmiumtelluride together vwith a quantity of agrouplla telluride and freezingrthe mixturethus formed. Thisxmethod ,is generallysimilarto the method heretofore describedin -connectionawith doping cadmium tellurideyvwith ,antimonv A .gtell.ur ide,such, as ...sodium telluride isV utilized in place of antimony; and the process may be carried ,out Ain an exactly. sirnilammanner. The alkalis are relatively diflcult tohandle in Itheir; pure; states and may beY more ,conveniently4 used in the .practiceofthe inventionin -the form` of. theintellurides.

Ptyvp e` semi-.conductive` cadmium ltelluride may be y pro duced` .according to the ,inventionhaving",fromf about 1014 to i018 impurity centers per cubic centimeter-of volume. The particular, concentration desired, will depend ,upon the .resistivity desired-inthe material, l,vs /hich in turn depends to alargeextent.uponI-theparticular use to` which the material .is to lbefput.l For certain .devices suchas those designed tor-operateat quenceshrfor examplegit maybe esirable to lprovide semigconductive cadmium telluride, of l:relativelyL ilotw .i =re s istivity. Othendevices -may ,require .relatively highnesistivity material. The resistivity.varies inversely..with the concentration of impurity centers. The concentra elativdyhish. ne.

tion of impurity centers may be controlled by varying the amount of impurity-yielding material incorporated in the melt in much the same manner as in the production of other semi-conductive materials such as germanium.

Crystalltes may be cut from selected portions of an ingot, produced as described heretofore, to provide semiconductive wafers consisting essentially of single crystal structures. Sufch wafers having p-type semi-conductivity mayvbe used to make semi-conductive devices such as those-described in my co-pending application, Serial No. 394,383-1iled concurrently herewith.

The .p-type impurity-yielding materials `described heretofore may be utilized as electrode materials in conjunction with n-type semi-conductive cadmium telluride to form semi-conductor devices having p-n rectifying junctions. These materials, especially those of column Va, which are relatively more stable than those of column la, may be utilized with cadmium telluride-to form alloy junctiondevices such as, for example, kthe device 'shown in Figure 3.

As shown in Figures 2 and 3, a pellet 16 of animpurity-yielding material selected from group Va, for example, antimony is .placed on :the surface of a waferlS of n-type semi-conductive cadmium telluride. .The

i wafer is preferably a single crystal and may be .of any convenientsize such as about 1/8 X 1/s" X .01 thick. The toeliet-may be of any size smaller 'than the wafer. It may conveniently be made in theform of a disc about .05 in diameter and..005 thick. The wafer andpellet are heated togetherin a non-oxidizing atmosphere at about 400-600 C.- for about rive minutes to alloy the pellet into the wafer and to form the device-shown in Figure 3.

The device is generally similar in eonstructional de tails to corresponding alloy type devices made of other semi-conductive materials. lt consists of the waferit of n-type semi-conductive cadmium telluride, and an antimony electrode le formed from the pellet 16 and fused to the surface of the wafer. The farthestpoint of penetration ofthe pellet into the Wafer is called 'the alloy front and is shown by the line 22. During the alloy process a portion of the wafer is dissolved into the molten pellet and during cooling a recrystallized region 20 rich in antimony and an integral part of the wafer is formed adjacent the alloy front. This region has. ptype conductivity. A p-n rectifying junction 24. is formed adjacent the alloy front. Electrical leads 26 and 28 may be attached by non-rectifying solder connections 27 and 29 to the electrode and the wafer respectively. The device may be etched, mounted and potted'according to the conventional techniques utilized iny conjunction with germanium semi-conductor devices.

Similar devices may be formed utilizing pelletsof'ia-ny of lthe-other elements heretofore described, mixturesof these elements'uor. alloys comprising significant-proportions of them. .ln general, because of their relatively high reactivity the elements of column la are Vleastconvenient to use, but alloys comprising themrnay be satisfactorily utilized.

There have Ythus been described improved semi-conductive materials, devices utilizing these materials, and improved methods of making both the materials vand, the

i devices.

which said impurity centers consist essentially of antimony atoms.

3. A semi-conductor device according to claim 1 in which said impurity centers consist essentially of lithium atoms.

4. In an alloy junction type semi-conductor device including a body of n-type semi-conductive material, a fused electrode alloyed to the surface of said body and a p-n rectifying junction disposed within said body adjament said electrode, the improvement comprising said materials being cadmium telluride and said electrode comprising a material selected from the Ia and Va columns of the Periodic Table according to Mendcleef which imparts p-type conductivity to semi-conductive cadmium telluride when dispersed therein.

5. A p-type semi-conductive material consisting essentially of cadmium telluride and having trace amounts of impurity centers dispersed throughout its mass, said impurity centers consisting of predominantly of atoms of at least one element selected from the Ia and Va columns yof the Periodic Table according to Mendeleef.

6. A material according to claim 5, in which said impurity centers consist predominantly of antimony atoms.

References Cited in the le of this patent UNITED STATES PATENTS 2,615,060 Martinace et al Oct. 21, 1952 2,629,672 Sparks Feb. 24, 1953 2,644,852 Dunlap `Tuly 7, 1953 2,697,052 Dacey et al. Dec. 14, 1954 2,719,799 Christian Oct. 4, 1955 2,743,199 Hull et al. Apr. 24, 1956 OTHER REFERENCES Chemical Abstracts (1952), v. 46, 38201-38201b. 

1. A SEMI-CONDUCTOR DEVICE COMPRISING A BODY OF SEMICONDUCTIVE CADMIUM TELLURIDE HAVING A P-TYPE SEMI-CON DUCTIVE REGION THE CONDUCTIVITY OF SAID REGION BEING PRIMARILY DETERMINED BY THE PRESENCE IN SAID REGION OF TRACE AMOUNTS OF UMPLURITY CENTERS CONISTING OF ATOMS OF AT LEAST ONE ELEMENT SELECTED FROM THE IA AND VA 